The flap I was getting other day while driving with a strong crosswind wasn't so much a rapid front-back up-and-down oscillation. It was as if the wind at that speed was lifting the hood from the side and creating the flap. Strange, as the hood alignment (gaps and height) are flush and even. I could only surmise that in my occurrence it was created by air spilling into the engine bay (from where I don't know) and actually lifting the hood on that edge from the underside. I'm puzzled further by the fact that the front wheel wells look fairly well sealed and the car has a pan under the engine compartment.

Perhaps it's just as well. Going *really* fast in strong crosswinds is probably not too intelligent an idea anyway...